Sound

Abstract

Originally, sound was defined as everything that was heard; i.e., periodic or nonperiodic vibrations of air in the frequency range of the human ear. But in addition to air, all gases, liquids, and solids can conduct similar types of vibrations that can be perceived as sound either by direct coupling to the human ear or with air as the coupling medium. The vibrations thus defined as sound are longitudinal vibrations, i.e., the particles move in the direction of propagation of the sound waves. The relative displacement of the particles within the sound wave generates a small change of pressure and density; therefore, these air-like vibrations which occur in fluid media are also called “dilatational vibrations.” In solids we also observe transverse vibrations (shear vibrations), i.e., the particles move transversely to the direction of propagation. Transverse waves represent the propagation of shear stresses; they do not affect the pressure or the density of the medium. However, the human ear responds also to periodic particle displacements, and it seems reasonable to interpret shear waves or transverse vibrations as a special case of a sound motion. Today we define as sound any vibration of a solid, liquid, or gaseous medium in the frequency range of the human ear, i.e., between about 16Hz and 16kHz Vibrations below 16Hz are called “infrasound;” those above 16kHz are called ”supersonic sound.“ The very high frequencies that appear in the mechanical spectrum of shear are usually called “hypersound.”